The present invention relates to systems for the transmission of rotary motion from a power source to a driven element, and more specifically to a tensioning assembly used in conjunction with a power transmission system to supply tension to a belt interconnected between the power source and the driven element.
In many applications, it is necessary to transfer the rotary motion of an output shaft connected to a power source from the output shaft to a driven element spaced from the power source. To accomplish this, normally a chain or belt is positioned around a first member, such as a pulley, fixed to the output shaft of the power source, and around a second, similar member fixed to a rotatable input shaft for the driven element.
In those circumstances where the power source is spaced a sizeable distance from the driven element, it is more cost effective to utilize a belt to operably connect the output shaft of the power source to the input shaft of the driven element. This is because a belt is easy to install, does not require lubrication, is very clean in operation, has lower maintenance, repair and/or replacement costs, and is able to dampen the shock loads transmitted by the belt between the power source and the driven element.
However, one downside to the use of a belt with this type of power transmission system arises due to the types of material that are used to form the belt. More specifically, the belt is normally constructed of a flexible material, such as a rubber, that enables the belt to be formed in an endless or looped shape that is readily positioned in engagement with the output and input shafts. While these types of materials for forming the belt are very tough and do not break easily, over time the belt will stretch due to an inherent stretch factor present in the material. This enables the belt to loosen and slip with regard to the rotation of the pulleys on the output and input shafts, such that the power from the power source can be sporadically, or not effectively transferred via the belt to the driven element.
In order to compensate for the stretching of the belt, one solution that has been developed involves mounting the power source to a sliding support which allows the power source to be moved with respect to the driven element to properly tension the belt. More specifically, as the belt stretches over time, the power source can be moved away from the driven element in order to compensate for the stretching of the belt.
An alternative solution developed to solve the above problem is to mount an idler to the power source that can movably contact and selectively apply tension to the belt. An example of a mechanism of this type is illustrated in Nelson U.S. Pat. No. 4,011,767, which is incorporated herein by reference. In this mechanism, the idler is pivotally secured on or adjacent the power source and is biased by a spring into contact with the belt. The tension applied to the idler by the spring is sufficient to approximately maintain a desired level of tension on the belt during various operating conditions of the power source.
However, in applications where the power source is quite large, i.e., approximately 500 HP and above, the design of the prior art idlers does not enable them to effectively and properly tension the belt by themselves as the magnitude of the forces required to be applied to the belt is too large for a spring-biased idler to effectively tension the belt. Thus, with these large power sources, to solve the problem of belt slippage, the first solution is utilized in that the power source is usually mounted to a base capable of sliding with respect to the fixed position of the driven element. This enables the power source to be moved a specified distance away from the driven element in order to properly tension the belt extending around the power source output shaft and the driven element input shaft.
However, while moving the power source away from the driven element effectively tensions the belt, other problems arise when using these large power sources. More specifically, in applications where the size of the power source utilized is large, the amount of belt stretching and the loads applied to the shafts on the power source and driven element, and the bearings connecting the shafts to the power source and driven element are greatly affected by the amount of power transmitted from the power source to the driven element by the belt. For example, when utilizing a large power source, extreme pulling forces are exerted by the belt on the bearings and shafts connected to the power source and driven element as a result of the operation of the power source and the driven element. These pulling forces can misalign the shafts with respect to their respective components, consequently shifting the bearings causing friction and heat and lubrication problems with the bearings positioned around the rotating shafts.
In order to overcome the shaft and bearing misalignment problems associated with the use of these larger power sources, a shaft extension or jack shaft can be positioned between the connection point of the belt to the power source, i.e., the pulley, and the power source itself. Most often the jack shaft is rigidly but rotatably mounted to pillow blocks adjacent the power source and is coupled at one end to the output shaft of the power source. The rigid structure of the jack shaft is such that any pulling forces exerted by the belt on the jack shaft are dissipated by the jack shaft and pillow blocks and are not transmitted to the output shaft, thereby preventing any misalignment of the output shaft or bearings. Therefore, the jack shaft provides enhanced strength and rigidity to the output shaft of the power source, such that the pulling forces or loads applied to the pulley and the jack shaft by the belt will not affect the alignment of the jack shaft and output shaft with respect to the power source. The same is true when the input shaft of the driven element is connected to a jack shaft as well.
However, while the presence of a jack shaft greatly reduces the occurrence of any misalignment of the respective shafts and pulleys, the size of the jack shaft used with the large power source is necessarily quite large itself, thereby increasing the overall size and weight of the apparatus that needs to be slidably mounted with respect to the driven element in order to properly tension the belt. As a result, larger mechanical forces are required to slide both the power source and the jack shaft, resulting in a greater cost for the overall power transmission system including these components.
In order to keep costs for power transmission assemblies of this type down, it is desirable to develop a power transmission system including a belt tensioning assembly compatible with a large power source that does not require the movement of the power transmission system with respect to the driven element. It is also desirable to develop a belt tensioning assembly that automatically controls the tension on the belt to both avoid slippage of the belt with respect to the pulleys, and misalignment of the output and input shafts and associated bearings with respect to the power source and driven element either in addition to or without a jack shaft.
It is an object of the present invention to provide a belt tensioning assembly for a power transmission system capable of tensioning a belt connected to the transmission system without having to slide or otherwise move the power source relative to a driven element.
It is another object of the present invention to provide a belt tensioning assembly for a power transmission system that is biased to constantly and automatically adjust the force of the tensioning assembly exerted on the belt during the operation of the power transmission system to maintain a specified amount of tension on the belt.
It is another object of the present invention to provide a belt tensioning assembly that automatically adjusts the force of the tensioning assembly on the belt when the output of the power source changes dramatically in order to compensate for sharply increasing or decreasing loads applied to the shafts and bearings of the system, making the use of a shaft extension or jack shaft connected to the output shaft of the power source optional within the radial load limits of the power source, and to eliminate the misalignment of the shafts with the power source and the driven element and the generation of heat within the bearings.
It is still another object of the present invention to provide a belt tensioning assembly for a power transmission system that allows for a reduction in the size of the bearings and shafts connected to the power source and to the driven element.
It is still a further object of the present invention to provide a power transmission system including a belt tensioning assembly that has a simple construction, allowing for inexpensive maintenance and easy replacement of any damaged and/or worn parts.
The present invention is an automatic belt tensioning assembly located adjacent to a power transmission system having a power source with an output shaft, a driven element with an input shaft, a pair of pulleys fixed to the output shaft and input shaft, and an endless belt disposed around the pulleys. The tensioning assembly includes a frame pivotally secured on or adjacent the power source or engine for the power transmission assembly and an idler roller rotatably attached to the frame opposite the power source. The idler contacts the slack side of the belt extending between the pulleys on the power source and the driven element, respectively. The idler is biased to deflect the belt in order to apply a proper amount of tension to the belt in order to optimize the transmission of power from the power source to the driven element depending upon the operating conditions of the power source.
The amount of force applied to the belt by the idler is controlled by a self-adjusting biasing mechanism attached between the power transmission system frame and the idler frame. The biasing mechanism urges the idler into contact with the belt, such that the idler can contact the belt with varying amounts of force depending upon the force applied by the belt to the idler in opposition to the bias of the adjustment mechanism against the belt. More specifically, the biasing mechanism includes a tension shaft pivotally mounted adjacent the power source but spaced from the pivot frame. The shaft is also pivotally mounted opposite the power source to the pivot frame using a guide block pivotally attached to the pivot frame and fixed to a sleeve slidably disposed on the tension shaft. The guide block is engaged by one end of a biasing member that is engaged at the opposite end by a cap fixed to a top end of the tension rod. As the idler is moved up and down based on the varying amounts of tension present in the slack side of the belt, the tension rod pivots with the idler such that the biasing member is either compressed or expanded, which allows the biasing member to in turn place more or less tension on the belt through the idler as necessary.
Various other features, objects and advantages of the invention will be made apparent from the following detailed description taken together with the drawings.